This is small enough that we can consider it to be zero for most practical applications. Our sensitivity equations are now:

(17)

(18)

(19)

(20)

(21)

Note that the Q sensitivity to R₁ and R₃ is zero, but the sensitivity to K is not. This may seem odd as it is R₁ and R₃ that set the gain. But that is only in the ideal case of high loop gain. As the loop gain of the circuit decreases with frequency, the gain eventually deviates from that set by the two resistors.

With an ideal operational amplifier (op amp) connected as a follower, K will always be precisely one. But a real op amp has finite open-loop gain. At frequencies with sufficient loop gain, reduction of K will be negligible. At higher frequencies the loop gain will be low enough that reduction in K will noticeably affect Q.

So, as long has we have chosen op amps with sufficient open-loop gain over our frequency range, we can assume K=1, and the equations further simplify to:

(22)

(23)

(24)

(25)

(26)

We have reduced all sensitivities to half or zero, except for Q sensitivity to K--the parameter that varies the least. You can't do much better than that!

What we have done, in a sense, is similar to noise shaping in a sigma-delta data converter where we move most of the noise out of the band of interest. Here we move the sensitivity into the inherently most stable parameter.

Note that the equation for Q has also simplified to:

(27)